Page 30 - Mechanics Analysis Composite Materials

Chapter 1. Introduction 15

Before being used as reinforcing elements of advanced composites, the fibers are
subjected to special finish surface treatments undertaken to prevent the fiber
damage under contact with processing equipment, to provide surface wetting when
fibers are combined with matrix materials, and to improve the interface bond
between fibers and matrices. The most commonly encountered surface treatments
are chemical sizing performed during the basic fiber formation operation and
resulting in a thin layer applied to the surface of the fiber, surface etching by acid,
plasma or corona discharge, and coating of fiber surface with thin metal or
ceramic layers.
With only a few exceptions (e.g., metal fibers), individual fibers, being very thin
and sensitive to damage, are not used in composite manufacturing directly, but in
the form of tows (rovings), yarns, and fabrics.
A unidirectional tow (roving) is a loose assemblage of parallel fibers consisting
usually of thousands of elementary fibers. Two main designations are used to
indicate the size of the tow, namely the K-number that gives the number of fibers in
the tow (e.g., 3K tow contains 3000 fibers) and the tex-number which is the mass in
grams of 1000 m of the tow. The tow tex-number depends not only on the number
of fibers but also on the fiber diameter and density. For example, AS4-6K tow
consisting of 6000 AS4 carbon fibers has 430 tex.
A yarn is a fine tow (usually it includes hundreds of fibers) slightly twisted (about
40 turns per meter) to provide the integrity of its structure necessary for textile
processing. Yarn size is indicated in tex-numbers or in textile denier-numbers (den)
such that 1tex =9den. Continuous yarns are used to make fabrics with various
weave patterns. There exist a wide variety of glass, carbon, aramid, and hybrid
fabrics whose nomenclature, structure, and properties are described elsewhere
(Peters, 1998; Chou and KO,1989; Tarnopol’skii et al., 1992; Bogdanovich and
Pastore, 1996).
An important characteristic of fibers is their processability that can be
evaluated as the ratio, Kp = Os/@., of the strength demonstrated by fibers in the
composite structure, OS, to the strength of fibers before they were processed, 0.
This ratio depends on fibers’ ultimate elongation, sensitivity to damage, and
manufacturing equipment causing the damage of fibers. The most sensitive to
operational damage are boron and high-modulus carbon fibers possessing
relatively low ultimate elongation 5 (less than 1%, see Fig. 1.7). For example,
for filament wound pressure vessels, K,= 0.96 for glass fibers, while for carbon
fibers, Kp= 0.86.
To evaluate fiber processability under real manufacturing conditions, three simple
tests are used -tension of a straight dry tow, tension of tows with loops, and tension
of a tow with a knot (see Fig. 1.10). Similar tests are used to determine the strength
of individual fibers (Fukuda et al., 1997). For carbon tows, normalized strength
obtained in these tests is presented in Table 1.3 (for proper comparison, the tows
should be of the same size). As follows from the Table, the tow processability
depends on the fiber ultimate strain (elongation). The best processability is observed
for aramid tows whose fibers have high elongation and low sensitivity to damage
(they are not monolithic and consist of thin fibrils).